1. Field of the Invention
The present invention relates to an internal combustion engine control having an exhaust gas purifying catalyst and, more particularly, to a catalyst warm-up control.
2. Related Art
An exhaust gas purifying catalyst, as generally mounted on a vehicle, subjects the noxious components (HC, CO and NO.sub.x) in the exhaust gas to oxidizing/reducing reactions in the state of a high temperature thereby to make them innoxious. In order to exhibit the exhaust gas purifying ability effectively, it is necessary to raise the temperature of the catalyst to an activation temperature (generally about 300 to 350.degree. C.). As a result, till the catalyst temperature rises to the activation temperature level after the engine start, the exhaust gas purifying ability is so low that many noxious components in the exhaust gas are discharged.
In order to solve this problem, there is in the prior art a technique for executing a fast catalyst warm-up control when the cooling water temperature at the engine start is low. As the fast catalyst warm-up control, proposed is a technique in which the exhaust gas temperature is raised to raise the catalyst temperature by delaying the ignition timing of the engine and by increasing the idling rotation speed, as disclosed in Japanese Patent Laid-Open No. 88870/1985. According to this fast catalyst warm-up control, however, the ignition delay angle lowers the engine torque and degrades the drivability. In order to warm up the catalyst with the heat of the exhaust gas, moreover, the idling rotation speed has to be raised to increase the exhaust gas amount so as to shorten the catalyst warm-up time period. This causes a problem that the emission and fuel mileage during the fast catalyst warm-up are deteriorated.
As the technique for raising the catalyst temperature by raising the exhaust gas temperature, on the other hand, there is a technique which is disclosed in Japanese Patent Laid-Open No. 508409/1994 (or U.S. Pat. No. 5,425,233). According to this technique, the exhaust gas temperature is raised by enriching the fuel injection amount to increase the HC and CO in the exhaust gas, by mixing the exhaust gas with the secondary air, as introduced into the exhaust pipe upstream of the catalyst, and by igniting the mixture with the spark ignition device. According to this technique, there are necessary a secondary air introduction pipe for introducing the secondary air to burn the enriched exhaust gas, an electric pump and a solenoid valve. This necessity complicates the construction and causes a problem that the cost is raised.
As disclosed in Japanese Patent Laid-Open No. 308311/1992 (or U.S. Pat. No. 5,315,823 and U.S. Pat. No. 5,211,011), there is a technique in which the catalyst is warmed up with the heat of reaction by executing an injection dither control for correcting the increase/decrease in the fuel injection amount to enhance the oxidations of HC and CO in the catalyst. In the fast catalyst warm-up control according to this injection dither control, the oxidations of HC and CO in the catalyst are promoted to warm up the catalyst with the heat of reaction. As a result, the oxidations of the HC and CO in the catalyst are not promoted while the catalyst is cold, to delay the temperature rise of the catalyst. This delay causes a problem that the emission in the fast catalyst warm-up is deteriorated.
As the technique for warming up the catalyst by making use of the oxidations in the catalyst, moreover, the fuel injection amount is enriched or increased, and the secondary air is introduced from the outside into the exhaust pipe upstream of the catalyst, as disclosed in Japanese Patent Laid-Open No. 171973/1993 (or U.S. Pat. No. 5,315,823). According to this technique, too, the secondary air is required to provide the secondary air introduction pipe for introducing the secondary air, the electric pump and the solenoid valve. As a result, the construction is complicated to cause a problem that the cost is raised.